Blower doors measure the amount of leakage in a home. It is comprised of a calibrated fan, a mounting system to attach the fan to an exterior door, and a manometer which measures pressure.
The principle behind a blower door test is simple: imagine a large parade balloon like the Spongebob balloon above. If the balloon is completely airtight we can pressurize it, shut off the valve and the balloon will remain inflated indefinitely.
Now imagine that the balloon has some small leaks at the seams. To keep it inflated, we need to continuously blow in air to replace the air that leaks through the seams. The larger the leaks, the more air is required. So, if we can measure the amount of air we blow into the balloon to keep it fully inflated, we can infer how leaky the balloon really is.
And that is what a blower door test does. It measures the amount of air that’s needed to keep a house at an elevated pressure of 50 Pascal (i.e. “inflated”), and we use this measurement to check how many leaks are present.
The blower door results can be expressed in a few different metrics. The most common one is air changes per hour (ACH), or how many times a house’s air completely replaced in a given hour. Since we take our blower door measurement at 50 Pascal, most codes and standards reference air changes at that elevated pressure (ACH50), but we can also calculate air changes under natural conditions (ACHn)
For example, a code-built new home with decent air sealing might have 7 air changes per hour at 50 Pascal (ACH50), meaning if we kept the blower door running for an hour it would pump in enough air to completely replace the home’s air 7 times. This would translate to about 0.35 natural air changes per hour (ACHn), or about one complete air replacement every 3 hours.
The metrics and math can get a little technical so let’s put them in context. Here’s a rough scale to compare your blower door test number to other standards:
10-20 ACH50 – In older homes, similar to “living in a barn”
7-10 ACH50 – Average new home with some air sealing but no verification and little attention to detail
7 ACH50 – OK infiltration level and the 2009 IECC energy code requirement
3-5 ACH50 – Good and achievable target for most new homes. The ENERGY STAR reference home is 5 ACH50 for climate zone 4 which covers DC, MD, VA and part of PA. The majority of PA is 4 ACH50 for the ENERGY STAR reference home.
3 ACH50 and lower – Tight home with great air sealing, and required by the 2012 energy code adopted in MD and coming soon to other jurisdictions.
.6 ACH50 – Super tight home and the Passive House standard.
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